Decoding Cyclic Voltammograms: Anodic and Cathodic Peaks

Welcome back. In our last lesson, we explored the fundamental working principle of cyclic voltammetry, understanding how a triangular potential waveform produces the characteristic "duck-shaped" voltammogram. We established that the peaks in a CV arise from the interplay between potential-driven reaction kinetics and mass-transport limitations.

Now that we understand why peaks appear, we can focus on what they tell us. This lesson will equip you to identify and interpret the anodic and cathodic peaks for simple redox processes. Your experience with electrochemical testing means you've seen countless such peaks in your data. Our goal here is to move beyond simple observation and learn how to extract quantitative thermodynamic and kinetic information from their position, height, and shape. This skill is the foundation for using CV as a powerful diagnostic tool in your electrocatalyst research.

The Anatomy of a Voltammogram

Let's begin by defining the key parameters we can extract from a typical cyclic voltammogram for a simple, reversible one-electron redox process, such as the oxidation and subsequent reduction of a species.

This idealized voltammogram clearly shows four primary parameters:

Anodic Peak Potential ( $E_{p a}$ ): The potential at which the maximum oxidation current is observed.
Anodic Peak Current ( $i_{p a}$ ): The maximum current measured during the anodic (forward) scan. By convention, it's measured from the extrapolated baseline of the non-Faradaic current.
Cathodic Peak Potential ( $E_{p c}$ ): The potential at which the maximum reduction current is observed.
Cathodic Peak Current ( $i_{p c}$ ): The maximum current measured during the cathodic (reverse) scan, also measured relative to an extrapolated baseline.

The following resource provides a step-by-step walkthrough of a voltammogram, explaining how to identify these key data points.

Cyclic Voltammetry Basic Principles, Theory & Setup - Ossila

This guide from Ossila provides a detailed breakdown of a typical CV curve and clearly defines the key parameters we extract from it.

First, read the section titled Cyclic Voltammograms Explained. This provides a narrative of the entire CV scan, detailing the processes that lead to the anodic peak and the subsequent decay. Then, jump to the section How to Read a Cyclic Voltammogram. Pay close attention to the labeled diagram and the definitions for the anodic peak current/potential and cathodic peak current/potential. Note the important practical detail about how peak currents are measured relative to an extrapolated baseline.

Interpreting Peak Potentials: Thermodynamics

The potentials at which the peaks appear are directly related to the thermodynamics of the redox reaction. For a reversible redox couple, we can use the peak potentials to estimate the formal potential ( $E^{0^{'}}$ ). The formal potential is similar to the standard electrode potential but applies to the specific experimental conditions (electrolyte, pH, etc.). It represents the potential at which the concentrations of the oxidized and reduced species at the electrode surface are equal.

For a reversible system, the formal potential can be approximated as the average of the anodic and cathodic peak potentials. This value is often called the half-wave potential ( $E_{1/2}$ ):

E_{1/2} = \frac{E _{p a} + E _{p c}}{2} \approx E^{0^{'}}

This simple calculation gives you a direct thermodynamic measure for your analyte under your specific experimental conditions.

Interpreting Peak Shape: Kinetics and Reversibility

While the average of the peak potentials tells us about thermodynamics, their separation and relative heights tell us about the kinetics of the electron transfer. Based on these features, we can classify a redox process as reversible, quasi-reversible, or irreversible. This is one of the most powerful diagnostic capabilities of CV.

The following short video provides an excellent visual summary of how to distinguish these different kinetic regimes.

Cyclic Voltammetry: Interpretation of Simple Cyclic Voltammogram

This video clearly demonstrates how to extract key parameters from a CV and then use them to interpret the underlying redox mechanism.

First, watch the brief introduction from the beginning to the explanation of how to find E_{pa}, E_{pc} and calculate the formal potential. This will reinforce what we just discussed. Then, watch the main segment on interpretation from redox mechanisms. Pay close attention to the three criteria (peak current ratio, peak separation, and scan rate dependence) used to classify a reaction as reversible, quasi-reversible, or irreversible.

Let's formalize the criteria from the video:

Electochemically Reversible:
- The electron transfer kinetics are very fast, much faster than the rate of mass transport. The system is essentially always in equilibrium at the electrode surface, as described by the Nernst equation.
- Criteria:
  - The ratio of peak currents is unity: $∣ i_{p a} / i_{p c} ∣ \approx 1$ .
  - The peak-to-peak separation is constant and has a theoretical value of $Δ E_{p} = E_{p a} - E_{p c} \approx \frac{59}{n}$ mV at 25 °C, where n is the number of electrons transferred. For a one-electron process, this is ~59 mV.
Quasi-Reversible:
- The electron transfer kinetics are on a similar timescale to mass transport. This is a very common scenario in real-world systems.
- Criteria:
  - The peak current ratio is still close to unity: $∣ i_{p a} / i_{p c} ∣ \approx 1$ .
  - The peak separation is greater than the ideal value: $Δ E_{p} > \frac{59}{n}$ mV.
  - $Δ E_{p}$ increases as the scan rate increases.
Irreversible:
- The electron transfer kinetics are very slow. The reverse reaction is so slow that it may not be observed on the timescale of the CV scan.
- Criteria:
  - A reverse peak may be completely absent. If it is present, it will be significantly smaller and shifted. Thus, $∣ i_{p a} / i_{p c} ∣ \neq = 1$ .
  - The peak potential shifts significantly with scan rate.

The article we consulted earlier also provides an excellent discussion of these concepts.

[PDF] A Practical Beginner's Guide to Cyclic Voltammetry

This article provides a rigorous definition of electrochemical reversibility and irreversibility.

Please read the two short paragraphs that define electrochemical reversibility and irreversibility. Start with the paragraph defining reversibility, and then read the paragraph that begins Electrochemical Irreversibility.

Your work on Ni-Fe based catalysts for HER and OER will often produce complex voltammograms that don't look like these simple examples. You might see multiple peaks corresponding to different oxidation states of Ni or Fe, or peaks that are irreversible.

Cyclic voltammograms of a catalyst material showing multiple anodic (a₁, a₂) and cathodic (c₁, c₂) peaks, as well as the onset of the hydrogen evolution reaction (H). This illustrates the complexity seen in real catalytic systems.

By understanding the characteristics of simple reversible, quasi-reversible, and irreversible processes, you can begin to deconstruct these more complex CVs, assigning features to specific redox events and gaining insight into the kinetics of your catalytic system.

Interpreting Peak Currents: The Randles-Sevcik Equation

Finally, the magnitude of the peak current, $i_{p}$ , is also rich with information. For a reversible process, the peak current is described by the Randles-Sevcik equation:

i_{p} = (2.69 \times 1 0^{5}) n^{3/2} A D^{1/2} C ν^{1/2}

where:

$n$ is the number of electrons transferred
$A$ is the electrode area (cm²)
$D$ is the diffusion coefficient of the analyte (cm²/s)
$C$ is the bulk concentration of the analyte (mol/cm³)
$ν$ is the scan rate (V/s)

You don't need to memorize the equation, but understanding the dependencies is crucial for diagnostics. The two most important relationships for you as an experimentalist are:

Peak current is proportional to concentration ( $i_{p} \propto C$ ). This is the basis for using CV as an analytical technique to determine analyte concentration.
Peak current is proportional to the square root of the scan rate ( $i_{p} \propto ν^{1/2}$ ). This is the hallmark of a process controlled by diffusion. Plotting your experimental peak current versus the square root of the scan rate should yield a straight line passing through the origin. If it doesn't, it indicates that the process is not purely diffusion-controlled and might involve surface adsorption or other phenomena.

Conclusion

In this lesson, we have moved from simply observing the peaks in a cyclic voltammogram to interpreting them with scientific rigor. By analyzing the key parameters of the anodic and cathodic peaks, you can now diagnose the thermodynamics and kinetics of a simple redox process.

Key Takeaways:

A voltammogram is defined by four key parameters: $E_{p a}$ , $i_{p a}$ , $E_{p c}$ , and $i_{p c}$ .
The average of the peak potentials, $E_{1/2}$ , provides an estimate of the thermodynamic formal potential ( $E^{0^{'}}$ ).
The peak separation ( $Δ E_{p}$ ) and peak current ratio ( $∣ i_{p a} / i_{p c} ∣$ ) reveal the electron transfer kinetics, allowing you to classify a system as reversible, quasi-reversible, or irreversible.
For a diffusion-controlled process, the peak current ( $i_{p}$ ) is proportional to the analyte concentration and the square root of the scan rate, as described by the Randles-Sevcik equation.

Preview of the Next Lesson:

We have focused exclusively on the Faradaic current that creates the peaks. However, the non-Faradaic, or capacitive, current in the "flat" region of the voltammogram also contains invaluable information. In our next lesson, we will learn how to use this capacitive current to determine the electrochemically active surface area (ECSA) of a catalyst—a critical metric for normalizing activity and making meaningful comparisons between different materials in your research.

Recommended resources

The Anatomy of a Voltammogram

Interpreting Peak Potentials: Thermodynamics

Interpreting Peak Shape: Kinetics and Reversibility

Interpreting Peak Currents: The Randles-Sevcik Equation

Conclusion